Significant challenges are encountered in the fabrication of nanostructures, particularly structures at a length scale of 10 nm to 50 nm. It is possible to fabricate isolated or semi-dense structures at this scale with an advanced lithographic technique such as electron beam lithography, but the exposure tools are extremely expensive and optimization of photo-resist processing is non-trivial and may not be amenable to strict control of dimensions or roughness.
New processes and novel materials are required to make nanofabrication easier, cheaper, and more versatile. Block copolymers are interesting materials for use in nanofabrication because they microphase separate to form ordered, chemically distinct domains with dimensions of 10's of nm. The size and shape of these domains can be controlled by manipulating the molecular weight and composition of the copolymer. Additionally, the interfaces between these domains have widths on the order of 1-5 nm and can be controlled by changing the chemical composition of the blocks of the copolymers. An advantage of using block copolymer systems as templates is that linewidth, tolerances and margins, and line edge roughness are dictated by thermodynamics (molecular weight, the Flory-Huggins interaction parameters χ between the blocks of the copolymer). It is unclear whether standard resist processing, where performance depends on control of kinetic processes, will be applicable at the scale of 10's of nm.
The feasibility of using thin films of block copolymers as templates was demonstrated previously by Chaikin and Register, et al., Science 276, 1401 (1997). Dense arrays of dots and holes with dimensions of 20 nm were transferred from a thin film of poly(styrene-b-isoprene) to silicon nitride substrates. The perfection of ordering of domains extend over grain sizes of approximately 1 μm2. For many applications, macroscopic orientation of the copolymer domains over areas as large as several cm2 and registration of the domains with the substrate will be required. Thin films with macroscopically ordered domains are envisioned as having potential in several applications including nanowires, magnetic storage media, quantum devices, and photonic crystals. Strategies for inducing macroscopic orientation of copolymer domains in thin films have included: (1) the use of electric fields to orient cylindrical domains in asymmetric diblock copolymer films both parallel to the film along electric field lines and perpendicular to the film in hexagonal arrays, (2) the use of miscut silicon wafers as substrates to align thickness induced morphologies along the corrugations of the substrate, (3) the use of miscut silicon wafers with obliquely deposited Au stripes to promote alternating wetting of the blocks on the alternating Si and Au stripes and perpendicular orientation of lamellar domains, and (4) the use of sidewall constraints to induce long range ordering of spherical domains in asymmetric diblock copolymers.
One approach to inducing macroscopic orientation of the domains of block copolymers combines advanced lithographic techniques and the self-assembly of the block copolymer film. Organic imaging layers are patterned using lithographic tools, e.g., proximity x-ray lithography with a mask and extreme ultraviolet (EUV) interferometric lithography. Regions of the imaging layer that are exposed to radiation or electrons undergo a chemical transformation that alters the surface chemistry of the imaging layer. A thin film of a symmetric diblock copolymer is then deposited on the patterned imaging layer and annealed above the glass transition temperature of the blocks of the copolymer. During annealing, the lamellar domains of the copolymer film self-assemble such that adjacent regions of the chemically patterned surface are wet by the different blocks of the copolymer. The lamellae orient perpendicular to the plane of the film and amplify the surface pattern. After annealing, selective removal of one of the blocks results in a nanopatterned template that can be used for additive or subtractive processes for nanofabrication. This strategy has the advantages of achieving macroscopic orientation of the lamellar domains using parallel exposure tools and registration of the patterned film with the substrate. See, Richard D. Peters, et al., “Using Self-Assembled Monolayers Exposed to X-Rays to Control the Wetting Behavior of Thin Films of Diblock Copolymers,” Langmuir, Vol. 16, 2000 (published on web Apr. 7, 2000), pp. 4625-4631; Qiang Wang, et al., “Symmetric Diblock Copolymer Thin Films Confined Between Homogenous and Patterned Surfaces: Simulations and Theory,” Journal of Chemical Physics, Vol. 112, No. 22, 8 Jun. 2000, pp. 9996-10010; Tae K. Kim, et al., “Chemical Modification of Self-Assembled Monolayers by Exposure to Soft X-Ray in Air,” J. Phys. Chem. B., Vol. 104, 2000 (published on web Jul. 18, 2000), pp. 7403-7410.